This issue contributes to the discussion of the relation between individual and aggregate behavior. The basic feature of all the models presented is that they allow for direct interaction between the individuals. This, it is shown, leads to a number of interesting phenomena that are difficult to account for in standard models. The basic message is that the inclusion of heterogeneous interacting agents allows us to escape from the pitfalls associated with the reduction to a representative individual and furthermore to show that aggregates will, in general, have behavior that is different from that of the component individuals. Lastly, complex aggregate behavior is not necessarily the result of complicated individual reasoning, it can emerge from very simple rule following by individuals.
